Electrical Technology (DKT 213)
Laboratory Module
EXPERIMENT 1
TITLE: SINGLE PHASE TRANSFORMERS - TRANSFORMER
REGULATION
OBJECTIVES
1) To determine the voltage regulation of a transformer with varying loads and to
discuss capacitive and inductive loading on transformer regulation.
2) To produce load regulation curves based on voltage and current
measurements.
EQUIPMENTS
EMS Workstation Model 8110, Single Phase Transformer Model 8341, Resistive
Load Model 8311, Inductive Load Model 8321, Capacitive Load Model 8331,
Power Supply Model 8821 and Data Acquisition Interface Model 9062.
INTRODUCTION
The load on a large power transformer in a sub-station will vary from a small
value in the early hours of the morning to a very high value during the heavy
peaks of maximum industrial and commercial activity. The transformer secondary
voltage will vary somewhat with the load, and because motors, incandescent
lamps and heating devices are all quite sensitive to voltage changes, transformer
regulation is of considerable importance. The secondary voltage also depends
upon whether the power factor of the load is leading, lagging or unity. Therefore,
it should be known how the transformer will behave (its voltage regulation) when
connected to a capacitive, an inductive or a resistive load. Transformer voltage
regulation in percent determined with the following formula:
Voltage Regulation (%)
where
=
100 X ENL -EFL
EFL
ENL
EFL
is the no-load secondary voltage
is the full-load secondary voltage
The result (a percentage value) obtained gives an indication of transformer
behaviour under load. The smaller the voltage regulation percentage, the smaller
the secondary voltage variation with the load and the better the voltage
regulation. Note that ENL is measured with the secondary winding open while EFL
is measured when nominal current flows in the secondary winding.
Several factors affect a transformer’s operation. The resistance and inductive
reactance of its winding cause internal voltage drops that vary with the amount of
current flowing in the windings. If the secondary is lightly loaded, current through
the winding resistance and reactance is small and the internal voltage drops are
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Laboratory Module
not significant. As the load increases, current and internal voltage drops increase.
If a transformer were perfectly ideal, its windings would have neither resistance
nor inductive reactance to cause the voltage drops. Such a transformer would
have perfect regulation under all load conditions and the secondary voltage
would remain absolutely constant. But practical transformer coils are made of
real wire and thereby have resistance and inductive reactance. Therefore the
primary and secondary windings have an overall resistance R and overall
reactance X. The simplified equivalent circuit of a practical transformer with a 1:1
turns ratio can be approximated by the circuit shown in Figure 1.1. The actual
transformer terminals are P1, P2 on the primary side and S1, S2 on the
secondary side.
Figure 1.1 Simplified Equivalent Circuit of a Practical Transformer
In this equivalent circuit, the practical transformer is shown to be made up of an
ideal transformer in series with impedance consisting of R and X that represents
the imperfections of the transformer. When a load (Z) is connected to the
secondary winding terminals (terminals S1 and S2), a series ac circuit consisting
of the secondary winding of the ideal transformer R, X, and Z is obtained.
Analysis of this series ac circuit shows that when the load is either resistive or
inductive, the load voltage decreases continuously as the load increases (as the
secondary current increases). Furthermore, when the load is capacitive, the load
voltage increases to a maximum as the load increases from zero (no load
condition) and then the load voltage decreases as the load continues to increase.
PROCEDURE
CAUTION
High voltages are present in this laboratory exercise! Do not make or modify any
banana jack connections with the power on unless otherwise specified!
1. Install the Power Supply, Data Acquisition Interface, Single-Phase Transformer,
Resistive Load, Capacitive Load and Inductive Load modules in the EMS
Workstation.
2. Make sure that the main switch of the Power Supply is set to the O (OFF)
position, and the voltage control knob is turned fully counter clockwise. Set the
voltmeter select switch to the 4-N position.
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Laboratory Module
3. Ensure that the DAI LOW POWER INPUT is connected to the main Power
Supply, set the 24V-AC power switch to the I (ON) position.
4.
Display the Metering application.
5. Set up the transformer loading circuit shown in Figure 1.2. Ensure that all
switches on the Resistive, Capacitive and Inductive Load modules are open and
connect E1, E2, I1, I2 as shown in the figure. Different load values will be used to
examine how the secondary (load) voltage changes as transformer loading
changes.
Line Voltage (V)
240
Es (V)
240
R (Ω)
∞
Figure 1.2 Transformer With a Variable Load
6. Turn on the main Power Supply and adjust the main voltage control to obtain the
value of Es given in Figure 1.2. With no load on the transformer (all switches
open on the load module), click the Record Data button to enter the
measurements for EPRI, IPRI, ESEC and ISEC in the Data Table.
7. Adjust the switches on the Resistive Load module to successively obtain the
resistance values given in Table 1.1. For each resistance value, record the
measurements as in step 6. When all data values have been recorded, rotate the
voltage control fully counter clockwise and turn off the Power Supply.
Line Voltage
(V)
240
R, XL, Xc
(Ω)
4800
R, XL, Xc
(Ω)
2400
R, XL, Xc
(Ω)
1600
R, XL, Xc
(Ω)
1200
R, XL, Xc
(Ω)
960
Table 1.1 Values for R, XL and Xc
8. Display the Graph screen, select E2 as the Y-axis parameter and I2 as the X-axis
parameter. Click the Line Graph button to observe the curve of secondary
voltage versus current. What happens to the secondary voltage as the resistive
load increases, i.e. load resistance decreases?
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9. Calculate the voltage regulation using the no-load (R=∞) and full-load
(R=minimum value) output voltages.
100 (ENL –EFL)
EFL
=
__________ %
10. Use the Clear All button in the Data Table window to clear the data and then
replace the Resistive Load module in the circuit of Figure 1.2 with the Inductive
Load module.
11. Turn on the main Power Supply and adjust the main voltage control to obtain the
value of Es given in Figure 1.2. With no load on the transformer (all switches
open on the load module), click the Record Data button to enter the
measurements for EPRI, IPRI, ESEC and ISEC in the Data Table.
12. Adjust the switches on the Inductive Load module to successively obtain the
reactance values given in Table 1.1. For each reactance value, record the
measurements as in step 11. When all data values have been recorded, rotate
the voltage control fully counter clockwise and turn off the Power Supply.
13. Display the Graph screen, select E2 as the Y-axis parameter and I2 as the X-axis
parameter. Click the Line Graph button to observe the curve of secondary
voltage versus current. How does the secondary voltage vary as the inductive
load increases?
14. Use the Clear All button in the Data Table window to clear the data and then
replace the Inductive Load module in the circuit of Figure 1.2 with the Capacitive
Load module.
15. Turn on the main Power Supply and adjust the main voltage control to obtain the
value of Es given in Figure 1.2. With no load on the transformer (all switches
open on the load module), click the Record Data button to enter the
measurements for EPRI, IPRI, ESEC and ISEC in the Data Table.
16. Adjust the switches on the Capacitive Load module to successively obtain the
reactance values given in Table 1.1. For each reactance value, record the
measurements as in step 15. When all data values have been recorded, rotate
the voltage control fully counter clockwise and turn off the Power Supply.
17. Display the Graph screen, select E2 as the Y-axis parameter and I2 as the X-axis
parameter. Click the Line Graph button to observe the curve of secondary
voltage versus current. How does the secondary voltage vary as the capacitive
load increases?
18. What differences do you observe between the three load curves?
19. Ensure that the Power Supply is turned off, the voltage control is fully counter
clockwise and remove all leads and cables.
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Electrical Technology (DKT 213)
Laboratory Module
Name: _________________________ Matrix No.: _____________ Date: __________
CALCULATION
9.
100 (ENL –EFL)
EFL
=
Instructor Approval: _____________________________________ Date: __________
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Electrical Technology (DKT 213)
Laboratory Module
Name: _________________________ Matrix No.: _____________ Date: __________
RESULTS
7.
Primary Voltage
(E1)
V
Secondary Voltage
(E2)
V
Primary Current
(I1)
A
Secondary Current
(I2)
A
Table 1-1 Transformer with a variable resistive load
8.
Secondary Voltage versus Current
(Resistive Load)
Secondary Voltage (V)
250
200
150
100
50
0
0.05
0.10
0.15
0.20
0.25
Secondary Current (load current) (A)
Figure 1-1 Secondary voltage versus current (resistive load)
______________________________________________________________________
Instructor Approval: _____________________________________ Date: __________
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Electrical Technology (DKT 213)
Laboratory Module
Name: _________________________ Matrix No.: _____________ Date: __________
12.
Primary Voltage
(E1)
V
Secondary Voltage
(E2)
V
Primary Current
(I1)
A
Secondary Current
(I2)
A
Table 1-2 Transformer with a variable inductive load
13.
Secondary Voltage versus Current
(Inductive Load)
Secondary Voltage (V)
250
200
150
100
50
0
0.05
0.10
0.15
0.20
0.25
Secondary Current (load current) (A)
Figure 1-2 Secondary voltage versus current (inductive load)
_________________________________________________________________
Instructor Approval: _____________________________________ Date: __________
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Electrical Technology (DKT 213)
Laboratory Module
Name: _________________________ Matrix No.: _____________ Date: __________
16.
Primary Voltage
(E1)
V
Secondary Voltage
(E2)
V
Primary Current
(I1)
A
Secondary Current
(I2)
A
Table 1-3 Transformer with a variable capacitive load
17.
Secondary Voltage versus Current
(Capacitive Load)
Secondary Voltage (V)
300
250
200
150
100
50
0
0.05
0.10
0.15
0.20
0.25
0.30
Secondary Current (load current) (A)
Figure 1-3 Secondary voltage versus current (capacitive load)
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18. ___________________________________________________________________
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Instructor Approval: _____________________________________ Date: __________
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Electrical Technology (DKT 213)
Laboratory Module
Name: _________________________ Matrix No.: _____________ Date: __________
DISCUSSION
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CONCLUSION
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Instructor Approval: _____________________________________ Date: __________
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